Large digestion tanks have long been used for the treatment of sewage to render the sewage innocuous and capable of being discharged into the environment without serious pollution problems. Most especially in anaerobic digestion processes, floating covers are used which float on the liquid surface, in order to force materials such as grease, scum and certain solids, which might otherwise float on the surface to be submerged, and thus more easily digested.
Recently developed processes combine sludge material generated by aerobic treatment systems as the feed material into anaerobic systems. Aerobic treatment systems are often associated with foam problems in anaerobic digestion. This foam, which has a specific gravity generally in the range of from 0.3 to 0.7, cannot support the floating roof at the same level as the liquid phase, which has a specific gravity of approximately 1. As a result of the formation of foam, and the problem of floating a cover on the foam, it has been found that the cover will sink down into the foam and the foam will extrude between the floating cover and the sidewalls of the digestion vessel. Any ga removal means, which are generally located at the top of the floating cover, will also tend to be clogged by the foam as it passes into the gas flow channels.
This problem of gas exhaust interruption is especially important where the floating cover is expected to float at least partially above the surface of the liquid, supported by a combination of the buoyancy effect of any submerged portions of the cover and an elevated gas pressure maintained between the roof member and the liquid surface. These types of covers require the continuous monitoring of gas pressure in order to maintain the roof at a desired location above the liquid; the gas exhaust ports thus should not be clogged by the foam which would interfere with gas pressure control.
Gas pressure-supported covers generally also have a weighted stabilizing member extending into the liquid in the vessel, where the weighted member is also partially supported by the buoyancy of the liquid.